The present invention relates to a multichannel photomultiplier for multiplying electrons through each of a plurality of channels.
A multichannel photomultiplier 100 shown in FIG. 1 is well known in the art. A conventional photomultiplier 100 includes a photocathode 103a disposed on an inner side of a light-receiving faceplate 103. Electrons are emitted from the photocathode 103a in response to incident light on the photocathode 103a. A focusing electrode 113 includes a plurality of focusing pieces 123 for focusing electrons emitted from the photocathode 103a in each of a plurality of channels An electron multiplying section 109 includes a plurality of stages of dynodes 108 for multiplying the focused electrons for each corresponding channel. An anode 112 collects electrons multiplied in multiple stages for each channel to generate an output signal for each channel.
The inventors of the present invention discovered that the conventional photomultiplier 100 described above could not sufficiently distinguish optical signals for each channel in measurements of higher precision due to crosstalk.
In view of the foregoing, it is an object of the present invention to provide a photomultiplier capable of suppressing crosstalk between channels in order to improve the capacity for distinguishing optical signals of each channel.
In order to attain the above object, the present invention provides a photomultiplier including, a light-receiving faceplate; a wall section forming a vacuum space with the light-receiving faceplate; a photocathode formed inside the vacuum space on an inner surface of the light-receiving faceplate for emitting electrons in response to light incident on the light-receiving faceplate; a focusing electrode provided in the vacuum space and having a plurality of focusing pieces, each of the focusing pieces having a surface subjected to an antireflection process, each pair of adjacent focusing pieces defining a channel therebetween to provide a plurality of channels, the focusing electrode focusing an electron emitted from the photocathode on a channel basis; an electron multiplying section provided inside the vacuum space for multiplying electrons focused by the focusing electrode for each corresponding channel; and an anode provided within the vacuum space for generating an output signal for each channel on the basis of electrons multiplied for each channel by the electron multiplying section.
In the photomultiplier of the present invention having this construction, light incident on an arbitrary channel of the photocathode causes electrons to be emitted from the corresponding channel. The electrons are converged in each channel by the corresponding pair of adjacent focusing pieces and guided to the corresponding channel of the electron multiplying section to be multiplied. The anode outputs an output signal corresponding to the channel. By treating the surfaces of each focusing piece in the focusing electrode with an antireflection process, the focusing pieces can prevent the reflection of light if light penetrates the photocathode. This construction prevents the emission of electrons from the photocathode in response to the light reflected from the focusing pieces, and prevents the emitted electrons from entering another channel such as the adjacent channel.
By treating the surfaces of each focusing piece in the focusing electrode with an antireflection process, the present invention can prevent the reflection of light off these focusing pieces that can cause undesired electrons to be emitted from the photocathode. Hence, the present invention can suppress crosstalk and improve the ability to differentiate optical signals for each channel.
Here, it is preferable that an oxide film be formed over the surface of each focusing piece as the antireflection process. Since the oxide film does not reflect light, surfaces treated with an antireflection process can be formed easily and reliably.
Alternatively, a porous metal deposition layer can be formed on the surface of each focusing piece as the antireflection process. Since the porous metal deposition layer can also prevent the reflection of light, the surfaces of the focusing pieces can be treated for antireflection easily and reliably.
The electron multiplying section includes a plurality of stages of dynodes, and each stage of the dynodes has a plurality of secondary electron multiplying pieces corresponding to each of the plurality of channels. When the plurality of stages of dynodes are arranged in sequence between the focusing electrode and the anode, it is preferable that the surfaces of a plurality of secondary electron emission pieces forming at least one stage of the dynodes in the line of sight of the photocathode are treated with an antireflection process.
Dynodes of stages positioned in the line of sight of the photocathode are positioned in direct view of the photocathode along a path extending linearly therefrom. Hence, light that penetrates the photocathode can strike the dynode. However, since the surfaces of each secondary electron emission piece in these stages of dynodes has been treated with an antireflection process, dynodes in these stages prevent the reflection of light that penetrates the photocathode. Hence, this construction prevents the emission of electrons in response to light being reflected back to the photocathode, thereby preventing such electrons from entering the adjacent channels. The construction can also prevent electrons from being emitted from the photocathode caused when unexpected light penetrates the photocathode and enters the adjacent channel, where the light is reflected by the dynodes as described above.
By performing an antireflection process on the surfaces of each secondary electron emission piece forming the dynodes of stages positioned in direct view of the photocathode, the present invention can prevent light from being reflected off these secondary electron emission pieces. Hence, the present invention can prevent the photocathode from emitting undesired electrons in response to the reflected light. As a result, the present invention can suppress crosstalk.
For example, when only the first stage dynode is positioned in direct line from the photocathode, the surfaces of each secondary electron emission piece forming the first stage dynode are treated with an antireflection process to prevent light from reflecting off of these secondary electron emission pieces. If both first and second stage dynodes are positioned in direct line from the photocathode, then the surfaces of each secondary electron emission piece forming the first and second stage dynodes are treated with an antireflection process to prevent reflection of light off of these secondary electron emission pieces.
Preferably, the electron multiplying section, for example, includes a plurality of stages of dynodes. Each stage of dynodes has a plurality of secondary electron multiplying pieces for the corresponding one of the plurality of channels. The stages of dynodes are arranged sequentially between the focusing electrode and the anode in order from a first stage to an n-th stage (n is an integer equal to or more than two). Each of the secondary electron emission pieces forms the first stage dynode having a surface subjected to an antireflection process.
With this construction, the surfaces of each secondary electron emission piece forming the first stage dynode has been treated with an antireflection process, thereby eliminating the reflection of light off of these secondary electron emission pieces and preventing the photocathode from emitting undesired electrons in response to such reflective light. Hence, the present invention can suppress crosstalk.
In this case, each secondary electron emission piece forming the second stage dynode may have a surface subjected to an antireflection process.
With this construction, the surfaces of each secondary electron emission piece forming the first and second stage dynodes has been treated with an antireflection process, thereby eliminating the reflection of light off of these secondary electron emission pieces and preventing the photocathode from emitting undesired electrons in response to such reflective light. Hence, the present invention can suppress crosstalk.
Here, it is preferable that an oxide film be formed over the surface of each secondary electron emission piece as the antireflection process. Since the oxide film does not reflect light, surfaces treated with an antireflection process can be formed easily and reliably.
Alternatively, a porous metal deposition layer can be formed on the surface of each secondary electron emission piece as the antireflection process. Since the porous metal deposition layer can also prevent the reflection of light, the surfaces of the focusing pieces can be treated for antireflection easily and reliably.
The electron multiplying section is preferably a layered type formed of a plurality of stages of dynodes in layers. Incident electrons can be reliably multiplied in each channel.
Preferably, the light-receiving faceplate includes a plurality of partitioning parts. Each of the partitioning parts corresponds to each one of the plurality of channels. The partitioning parts prevents light incident on one of the channels in the light-receiving faceplate from entering a channel adjacent to the one of the channels in the light-receiving faceplate.
By providing the partitioning parts to prevent light incident on one channel in the light-receiving faceplate from entering an adjacent channel, the present invention can further suppress crosstalk.
The partitioning parts are preferably formed of a light-absorbing glass, for example. Since the light-absorbing glass absorbs light incident on one channel that reaches the partitioning part, this construction can prevent light from entering the adjacent channels and can reliably suppress crosstalk.
The light-receiving faceplate preferably includes condensing means for condensing light incident on any position in each channel to a prescribed region in a corresponding channel of the photocathode when each pair of adjacent focusing pieces effectively focuses electrons emitted from the prescribed region within the corresponding channel of the photocathode and guides the electrons in the corresponding channel. The condensing means collects light incident on any position in a channel of the light-receiving faceplate to a prescribed region of the corresponding channel in the photocathode. Electrons converted from light at the prescribed region are reliably focused by the corresponding pair of adjacent focusing pieces and are guided and multiplied in the corresponding channel of the electron multiplying section. Hence, light incident on each channel is effectively multiplied.
The condensing means preferably includes a plurality of condensing lenses disposed on an outer surface of the light-receiving faceplate in a one-on-one correspondence with the plurality of channels.
When the condensing means has condensing lenses arranged on the outer surface of the light-receiving faceplate corresponding to each channel in this way, the condensing lenses can reliably condense light for each channel.
Alternatively, the condensing means may include a plurality of condensing lens-shaped parts formed on an outer surface of the light-receiving faceplate in a one-on-one correspondence with the plurality of channels.
By forming a plurality of condensing lens-shaped parts on the outer surface of the light-receiving faceplate itself, it is possible to condense light reliably for each channel through a simple construction.